Without a timer the defrost cycle is fixed. Any idea how many minutes the fixed cycle is for this defrost board?

Its not fixed. It only defrost when when it determines the coil is to frozen/frosted. One day it might do it once every 2 hours, the next once every 3 hours, and below 20 degrees not at all. It will defrost once every 6 hours to make sure the outdoor coil doesn't become oil logged. All times I posted are compressor run times, not just hours of the day.

On demand defrost is the more efficienct way to do defrost, as it doesn't do them needlessly.